a) Field of the Invention
This invention relates to a production process of a rolling bearing for a high-speed rotating equipment employed in a field where high speed and high safety are required, such as a medical, food-processing or aerospace equipment.
More specifically, the present invention is concerned with an efficient and economical production process of a porous retainer of the type that as a principal element of a rolling bearing, it is used in a form impregnated with a lubricating oil.
b) Description of the Related Art
Conventional examples of a retainer for holding rolling elements (balls) in a rolling bearing include those made of a self-lubricating molding material, for example, a synthetic resin such as polytetrafluoroethylene.
Further, as examples of an inner ring, an outer ring, rolling elements and a retainer in a bearing, there are those obtained by plating, coating or sputtering these members with a solid lubricant, for example, a metal such as gold or silver, or molybdenum disulfide.
It is however the current situation that a bearing element, for example, a retainer making use of the above-described self-lubricating synthetic resin or the above-described solid lubricant such as gold or silver is generally poor in the durability of a lubricant film and cannot stably exhibit sliding properties over a long time.
Especially in a lubrication system of a high-speed rotating equipment such as a high-speed cutter, it is strongly required to stably and continuously supply a lubricating oil onto a friction surface in a bearing for many hours and also to provide the friction surface with wear resistance. To the best of the inventors' knowledge, there is no retainer which can meet such needs.
A high-speed rotating equipment, especially a high-speed cutter for rotating a cutting tool at high speeds is generally constructed of a rotary shaft for fixedly holding various cutting tools thereon, a drive unit for rotating the rotary shaft, and a bearing unit rotatably supporting the rotary shaft.
As a high-speed cutter of this type, an odonto-therapeutic high-speed cutter (i.e., air turbine hand piece) can be mentioned by way of example.
Bearing units for the above-mentioned odonto-therapeutic high-speed cutters (i.e., air turbine hand pieces) are known to include those having a ball bearing system making use o f balls (rolling elements) and those equipped with a (non-contact) air bearing system making use of an air bearing.
Paying attention, for example, to the bearing mechanisms of dental air turbine hand pieces, two types of air turbine hand pieces are known, one being of the ball bearing turbine type and the other of the air bearing turbine type.
The former type, i.e., the ball bearing turbine type can be considered to be a high-speed rotation type for approximately 200,000 to 400,000 rpm, while the latter type, that is, the ball bearing turbine type can be considered to be a super high-speed rotation type for approximately 300,000 to 500,000 rpm.
It is however to be noted that the above-described revolution speeds of the ball bearing turbine type and the air bearing turbine type are general values. For example, the dental air turbine hand piece already proposed by the present inventors (U.S. Pat. No. 5,562,446) is of the ball bearing turbine type. Nonetheless, it has high performance so that super high-speed rotation can be achieved.
To facilitate the understanding of the conventional art and this invention, a description will now be made about the construction of an equipment to which a bearing making use of a porous retainer produced by the production process of this invention and a lubricating oil, is applied, namely, of a dental high-speed cutter (i.e., a dental air turbine hand piece).
FIG. 1 to FIG. 2 show the construction of the dental air turbine hand piece, in which FIG. 1 is a perspective view illustrating the overall construction and FIG. 2 is a cross-sectional view illustrating the internal construction of a head portion and a neck portion in particular.
As is depicted in FIG. 1, the dental air turbine hand piece designated generally by letter A is composed of a head portion H, which carries a cutting tool B(5) fixedly held on a rotor shaft (drive shaft) of an air turbine, and a grip portion G.
A neck portion N of the grip portion G is connected to the head H, and is internally provided with means for supplying compressed air to the air turbine arranged within the head portion H and also for discharging compressed air from the air turbine.
FIG. 2 illustrates the internal construction of the head portion H and the neck portion N of the dental air turbine hand piece A.
As is illustrated in the figure, in the head portion H, a turbine rotor shaft 3 with turbine blades 2 disposed at a peripheral edge portion thereof is arranged within a chamber 11 of a head 1, and the turbine rotor shaft 3 is rotatably supported within the head 1 by way of a bearing unit 4.
The head 1 is composed of a head main part 12 and a cap part 13. Within the head main part 12, the bearing unit 4 is arranged to rotatably support the turbine rotor shaft 3. To perform treatment, the cutting tool 5 is fixedly held in a bore which is formed through the turbine rotor shaft 3 along a central axis thereof. Incidentally, the cutting tool 5 is provided on a peripheral side wall thereof with a chuck 51 for holding the cutting tool 5 in place within the bore.
The bearing unit 4 is of the ball bearing type and is constructed of an inner ring 41, an outer ring 42, rolling elements 43 and a retainer 44. The bearing unit 4 may be provided on an outer periphery or side wall thereof with O-rings for providing the bearing unit with self-centering function and/or with known wave washers for enhancing the rigidity of the shaft.
A main part 6 of the neck portion N is provided with an air supply passage 7 and an air inlet 71 for supplying compressed air t o the turbine blades 2 arranged within the chamber 11 and also with air discharge passages 8,9 and air outlets 81,91 for discharging compressed air from the chamber 11.
In the above-described internal construction of the dental air turbine hand piece A as illustrated in FIG. 2, the means for supplying and discharging compressed air is the one already proposed by the present inventors (U.S. Pat. No. 5,562,446) and is of a new construction totally unseen in the conventional art.
Accordingly, FIG. 2 contains other reference symbols in addition to those referred to in the above to describe the individual elements (members). Although a description of these additional symbols is omitted herein, the construction of the conventional dental air turbine hand piece can be easily understood on the basis of FIG. 2.
The dental air turbine hand piece A--which is equipped with the air supply and discharge means shown in FIG. 2 and already proposed by the present inventors--belongs to the category of conventional hand pieces with a rolling bearing built therein, but makes it possible to obtain rotation of an extremely high speed and hence a large torque.
In the above-described dental air turbine hand piece of the ball bearing type, its bearing unit is in the form of a miniature bearing unit. Since the turbine rotor shaft rotates at a high speed of approximately 200,000 to 400,000 revolutions per minute, the temperature inside the bearing unit becomes high and further, the bearing unit is exposed to a large stress. For a lubricating oil to be applied to a bearing which is used under the above-described severe conditions, it is therefore extremely important to control its quality and properties.
Further, the above-described dental air turbine hand piece of the ball bearing type is used in the oral cavity. Accordingly, the dental air turbine hand piece is used by spraying or dropping a lubricating oil to the bearing unit, in other words, is used under an environment of minimum lubrication, and is subjected to high-pressure and high-temperature treatment for sterilization and disinfection (which is also called "autoclaving", which is applied under the following conditions: steam pressure, 2.4 kg/cm.sup.2 ; temperature, 135.degree. C.; time, 5 minutes).
As a rolling bearing for use in the above-described dental air turbine hand piece, one capable of meeting conditions similar to those mentioned above is therefore required. In particular, as a retainer which acts as an important element in a lubrication system, one capable of satisfying conditions similar to those mentioned above is required.
Further, without needing mentioning, the lubricating oil for use in the above bearing is also required to have properties sufficient to withstand such severe use conditions, such as oxidation resistance.
Conventional examples of the above-described retainer for rolling elements (balls) include those made of polyimide resins or fiber-layer-containing phenol resins from the viewpoint of the above-described property requirements. These conventional retainers are however not considered to be sufficient.
In view of the above-described severe use conditions for dental air turbine hand pieces, dental air turbine hand pieces equipped with retainers impregnated with fluorinated oils, which have excellent heat resistance, permit sterilization and disinfection (autoclaving) and have superb lubricity, have been proposed recently in Japanese Patent Publication (Kokoku) No. HEI 5-43884 and Japanese Utility Model Application Laid-Open (Kokai) No. HEI 7-10553.
Incidentally, the retainers are in the form of porous members obtained by sintering powder of a polyimide resin.
The fluorinated oils have properties such that they are inactive, are excellent in heat resistance, chemical resistance and solvent resistance and, even when exposed to high temperatures, do not form a solid deterioration material. It is therefore possible to consider that the above-described proposed dental air turbine hand pieces have used these properties for the retainers.
Concerning a rolling bearing for a high-speed rotating equipment, for example, for a dental air turbine hand piece of the ball bearing type, especially a retainer as a principal element of the rolling bearing, a variety of examples of different materials and structures have been developed in the light of required characteristics as described above.
For example, as a rolling bearing for a dental air turbine hand piece of the high-speed rotation system, especially as its retainer, a porous product obtained by sintering powder of a polyimide resin from the viewpoint of heat resistance (autoclaving resistance), durability and the like as mentioned above has been proposed.
As the above-mentioned porous retainer made of the polyimide resin is produced by compressing and sintering powder of a polyimide resin of a desired particle size (for example, 15 to 50 .mu.m) under desired conditions, it is however difficult to form a porous portion of a uniform communicated structure in the matrix material.
Needless to say, a failure in forming a porous portion of a uniform communicated structure in a matrix material (polyimide resin) means that the matrix material cannot be impregnated evenly with a lubricating oil. This uneven impregnation will then give serious adverse effects on lubrication characteristics in a high-speed rotation system.
In the production of a porous retainer by the above-described sintering of power of a polyimide resin, it is extremely difficult especially in providing the porous retainer with uniform quality. The production must therefore be performed under stringent conditions.